10-4-4, A Cardiac Glycoside Block Growth in Cancer Cells via Nuclear Receptor Nur77 Mediated Na+/K+- ATPase Endocytosis

Background Cancer is second only to heart disease as a cause of death. Develop new and more effective treatment strategies for cancer remain a major challenge for human medicine today. Nur77, an orphan member for the nuclear receptor superfamily, inhibits growth in cancer cells by translocation to cytoplasmic. Small molecules that trigger Nur77 nuclear export may be an ideal anti-cancer candidate. Methods Cell proliferation was evaluated by 3-(4, 5-dimethylthiazol-2-y1)-2, 5-diphenyl tetrazolium bromide (MTT) assay. The protein expression level were detected by western blot analysis. Immunostaining and cell fractionation assays were used to assess subcelluar localization of Nur77. Cell apoptosis, cell cycle and calcium were detected by flow cytometry. Zebrafish liver cancer models were used to determine anti-cancer effect of 10-4-4 in vivo. Results In this study, we exhibit 10-4-4 a cardiac glycoside, extracted from Antiaris toxicaria lesch, has sensitivity to cancer cells. 10-4-4 induces apoptosis and G2/M cell cycle arrest in HepG2 cells. Consistently, 10-4-4 augments Nur77 expression and cytoplasmic localization, its restraint of cancer cells growth is Nur77 dependent. Meanwhile, as a cardiac glycoside, 10-4-4 inhibits Na+/K+- ATPase (NaK) activation. To further confirm the molecular mechanism of 10-4-4, we found the association between Nur77 and NaK. The suppression of NaK by 10-4-4 increases the level of intracellular Ca2+. Ca2+, as a second messenger, specific activates protein kinase C (PKC). PKC has been reported on the influence of Nur77 nuclear export. Identical conclusions are obtained in this studies that 10-4-4 induces PKC activation play an important role in Nur77 nuclear export. Notably, the cytoplasmic Nur77 induced by 10-4-4 interaction with NaK to induce NaK endocytosis, and then trigger G2/M cell cycle arrest and apoptosis. Studies in Zebrafish shows that 10-4-4 potently inhibits the growth of liver cancer cells in vivo. Conclusions Our results exhibit that 10-4-4 possesses an anti-cancer activity in vitro and in vivo via NaK-Nur77 signaling pathway and maybe offers a novel strategy in development of chemotherapeutic anti-cancer drug.